A direct digital radio frequency (RF) transmitter (TX) has several advantages compared to the digital-analog-RF transmitters. The direct digital-RF transmitter shifts the digital-analog interface close to the antenna, and thus fewer analog components are involved. Typical analog issues like In-phase (I) and Quadrature-phase (Q) signals mismatch, local oscillator leakage, and image distortion can be largely alleviated and even avoided in direct-digital-RF transmitters. The direct digital-RF transmitter also enhances system flexibility through multi-mode and multi-band operation enabled by digital signal processing. In addition, the direct digital-RF transmitter can take advantage of the increasing speed and capacity of digital processing, as well as high-level integration. Moreover, the power amplification in digital transmitter is based on highly efficient switch-mode operation, improving the transmitter energy efficiency and environment. Thus, the direct digital-RF transmitters have benefits for both wireless base-station and mobile applications.
In order to increase a wireless data rate and to improve network coverage with the efficient use of spectrum, concurrent multi-band transmission methods have been developed. For example, the long-term evolution (LTE) communication standard defines non-contiguous carrier aggregation for concurrently transmitting multiple disjoint frequency bands. Accordingly, there is a need to generate a signal with multiple disjoint frequency bands and sufficient power for radio transmission.
Some methods generate multi-band signal simply by using multiple transmitters, i.e., each transmitter transmits RF signals on one frequency band. The reason for using multi-transmitters is a limited bandwidth that each transmitter can support. Particularly, when the multiple bands are not adjacent, i.e. non-contiguous, the transmitter bandwidth becomes a bottleneck. In conventional RF transmitters, the bandwidth is largely determined by the Q of RF power amplifier impedance matching network. It can be a difficult task to design broadband and efficient analog RF power amplifier. The multiple-transmitters architecture provides high performance by maintaining a high Q and transmitting each narrow band using a different transmitter. Although this multi-transmitter architecture is straightforward to implement, multi-fold increase in size and cost is inevitable.
Therefore, there is a need to provide a digital transmitter enabling concurrent non-contiguous multi-band operation.